linux-kernel - Re: [PATCH 1/2] block: add initial kdoc over the request

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Date:   Wed, 24 Jun 2020 00:33:56 +0000
From:   Damien Le Moal <Damien.LeMoal@....com>
To:     Luis Chamberlain <mcgrof@...nel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "bvanassche@....org" <bvanassche@....org>,
        "ming.lei@...hat.com" <ming.lei@...hat.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "satyat@...gle.com" <satyat@...gle.com>
CC:     "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] block: add initial kdoc over the request_queue

On 2020/06/24 7:03, Luis Chamberlain wrote:
> We start off with an initial description of the request_queue data
> structure, followed by describing the purpose of the debugfs_mutex
> debugfs_dir, and blk_trace.
> 
> Suggested-by: Bart Van Assche <bvanassche@....org>
> Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
> ---
>  include/linux/blkdev.h | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 70461b347169..ea319c2b0593 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -394,6 +394,26 @@ static inline int blkdev_zone_mgmt_ioctl(struct block_device *bdev,
>  
>  #endif /* CONFIG_BLK_DEV_ZONED */
>  
> +/**
> + * struct request_queue - block device driver request queue
> + * @debugfs_mutex: used to protect access to the @debugfs_dir
> + * @blk_trace: used by blktrace to keep track of setup / tracing
> + * @debugfs_dir: directory created to place debugfs information. This is always
> + *	created for make_request and request-based block drivers upon
> + *	initialization. blktrace requires for this directory to be created,

s/requires for/requires/ (may be, not sure of my English grammar on this one :))

> + *	and so it will be created on demand if its block driver type does not
> + *	create it opon initialization.

s/opon/upon/

> + *
> + * The request_queue is used to manage incoming block layer device driver
> + * requests. We have three main type of block driver types which end up making

We have three main types of block drivers which...

> + * use of the request_queue:
> + *
> + *   o make_request block drivers (multiqueue)
> + *   o request-based block drivers

Isn't this second one BIO based drivers, like device-mapper BIO targets ?

> + *   o custom solutions such as scsi-generic

The SG driver does not create a request queue, it uses the one of the scsi
device created by the scsi stack. Is this what you mean here ? Saying
scsi-generic kind of implies sg driver.

> + *
> + * All partitions share the same request_queue data structure.
> + */
>  struct request_queue {
>  	struct request		*last_merge;
>  	struct elevator_queue	*elevator;
> 


-- 
Damien Le Moal
Western Digital Research